Visbreaking processes for mild conversion of resid feeds are well known in the art. These processes are utilized to perform a thermal, usually non-catalytic, partial conversion of a heavy hydrocarbon stream into lighter hydrocarbon products. Preferred heavy hydrocarbon feedstream to the visbreaking process are those that have an initial boiling point above 600° F. (316° C.), more preferably above about 800° F. (427° C.). Preferred visbreaker feeds may be comprised of crude atmospheric tower bottoms, crude vacuum tower gas oils and/or crude vacuum tower bottoms.
Visbreaker feedstreams are generally comprised of high molecular weight paraffins, aromatics, asphaltenes, as well as aromatics and asphaltenes with paraffinic side chains. These feedstreams are usually highly viscous with viscosities generally from about 20 to about 1500 centistokes at 212° F. (100° C.). The visbreaking process can be utilized to thermally crack these highly viscous, high molecular weight hydrocarbons into lighter, less viscous products. Preferably, a significant amount of products can be converted into the naphtha boiling range products (boiling range of about 80° F. to about 450° F.), and distillate to gas oil range products (boiling range of about 350° F. to about 800° F.). However, excessive severity (i.e., conversion to lighter products) in a visbreaking process can lead to several problems. For a given unit and feedstream, the severity of the unit is generally a function of the temperature at which the feedstream leaves the visbreaker reactor.
Firstly, high severities can result in an overabundance of light gases generated from the visbreaking process. These light gas products are generally of low economic value and therefore undesired reaction products. Secondly, high severities can result in highly aromatic product streams. These highly aromatic product streams may be of limited value for use in commercial fuels products due to restrictions on aromatic fuel contents and may also cause the fuel products to be excessively unstable. These products may polymerize and develop waxes bringing the desired products out of required fuel specifications as well as causing pluggage problems in associated equipment.
Another more severe problem is that high severity of visbreaking can result in an excessive amount of coke formation in the visbreaking unit. Although facilities and operating conditions may minimize as well as remove some of the coke formation in the unit, the coke production and formation in the visbreaking units increases with increasing severity and operating temperature. As a result, visbreaker units must be taken out of service at periodic intervals in order to remove the coke that forms in the unit. Lower severity operations increases the available on-stream time of these units. Therefore, for the reasons above, it is desirable to run the visbreaker unit within a threshold severity and reactor outlet temperature.
Some visbreaker units include the use of a soaker drum between the visbreaking reactor and the visbreaker fractionator. The soaker drum allows the visbroken product stream leaving the visbreaking reactor to have additional residence time at the heated temperature prior to being quenched in the visbreaker fractionator. This additional residence time allows the visbreaker reactor to be run at a lower outlet temperature when achieving a similar conversion as to a visbreaker unit without a soaker drum. However, although the use of a soaker drum in the visbreaking process assists in reducing coke formation in the unit thereby obtaining longer on-stream intervals, this configuration does not generally result in significant improvement in the product stream composition.
Due to the limited severity that the visbreaker unit may run, there is still a large amount of the product from the visbreaker reactor that is in the heavy gas oil range (550° F. to about 800° F.) as well as visbreaker bottoms which generally have boiling points above 750° F. (399° C.), more typically above about 800° F. (427° C.).
A problem that exists is that the heavy gas oil range products from the visbreaker contain significant amounts of aromatic hydrocarbons. Although it is often desired to further catalytically crack these gas oil range materials into lighter fuels such as naphthas or gasolines, these highly aromatic feedstreams can result in excessive coke formation on the cracking catalysts (e.g., a fluid catalytic cracking or hydrocracking catalyst) resulting in decreased catalytic activity, as well as increased unwanted processing unit emissions (such as CO and CO2).
Similarly, the visbreaker bottoms product stream possesses similar undesirable properties due to its high aromatic content. However, in the visbreaker bottoms product stream a significant amount of the aromatic content of the stream is in the form of asphaltenes. The visbreaker bottoms product stream normally has a high Conradson Carbon Residue (CCR) number which indicates the amount of coke (carbon) that a certain stream will produce. The high asphaltene content and high CCR content of the visbreaking bottoms product stream render it prohibitive to further catalytically process this stream and therefore, the visbreaking bottoms product stream is usually thermally cracked in a resid conversion unit such as a coker unit or diluted as required for sale as fuel oils. The problem that exists is that both the visbreaker gas oil products and the visbreaking bottoms products contain a significant amount of valuable high molecular weight saturated hydrocarbons with relatively low CCR content in the product streams which cannot be removed from the undesired highly aromatic, high CCR hydrocarbons through conventional fractionation techniques. These captured saturated hydrocarbons would make very valuable feedstocks to the refinery catalytic cracking processes if there were a process to selectively segregate these molecules from the aromatic hydrocarbons feedstream components. Since they cannot be removed in conventional visbreaking or fractionation processes, a significant amount of these high value, upgradeable hydrocarbon components are lost in thermal conversion processes.
Therefore, there exists in the art a need to separate from select visbreaker product streams a high value hydrocarbon stream with reduced CCR content and increased saturated hydrocarbons content for use as a feedstream to refinery and petrochemical catalytic upgrading processes.